There has been widespread use of closed batteries in cylindrical or button-like shape as a power source of miniature electronic equipment, but there has been an increasing tendency in recent years towards mounting a closed battery in rectangular shape, which is deemed excellent in maximizing the use of space as compared with those closed batteries earlier described above, in various electronic equipment.
The closed batteries in rectangular shape include one fabricated by housing a spiral electrode in an outer case in the shape of a flat bottomed rectangle, having an opening on one side in the longitudinal direction thereof, and subsequently covering the opening with a sealing plate, thereby sealing the outer case by laser-welding the sealing plate to the outer case. In JP, H 10-162792, A, and JP, H 09-171809, A, a method of sealing the outer case has been disclosed whereby an electrode current collector tab is clamped between the rim of the opening and the sealing plate when welding the sealing plate to the opening such that the sealing plate is laser-welded to the rim of the opening and concurrently, the electrode current collector tab as well is welded to the outer case, thereby effecting electrical connection thereof with the outer case.
FIG. 3 is a perspective view showing the essential parts of a closed battery disclosed in JP, H 10-162792, A, in which a spiral electrode body 1 comprising a positive electrode, a negative electrode, and a separator interposed therebetween is housed in an outer case 2 of the battery, made of aluminum, doubling as a positive electrode outer terminal, and subsequently, a sealing plate 3 is laser-welded to the rim of the opening of the outer case 2.
A positive electrode current collector tab 4, leading from the exposed part of the core body of the positive electrode is clamped between the rim face of the sealing plate 3 and the opening of the outer case 2 at the time of the laser-welding, such that the sealing plate 3 is laser-welded to the outer case 2, thereby sealing the opening of the outer case 2, and at the same time, the positive electrode current collector tab 4 as well is welded to the outer case 2 to implement electrical connection therebetween. An incised part of a shape substantially resembling the letter U is formed at the winding end of the exposed part of the core body of the positive electrode, and a slender piece of foil, cut and raised from the incised part, is used as the positive electrode current collector tab 4.
Further, a negative electrode current collector tab 11 is separately extended from the spiral electrode body 1, and is electrically connected with a current collector terminal plate 13 while an adhesive tape (not shown) is attached to the winding end portion of the spiral electrode body 1 so as to maintain the spiral electrode body 1 in its current state. An adhesive tape 12 is adhered to the positive electrode current collector tab 4 so as to securely hold the latter.
However, since the positive electrode current collector tab 4 is made of a slender piece of foil, cut and raised from a portion of the exposed part of the core body of the positive electrode, the characteristics of the foil piece, such as mechanical strength, and so forth, are dependent on the material characteristics of the core body of the positive electrode. Because a piece of aluminum foil about 15 μm thick is normally used for the exposed part of the core body of the positive electrode, the positive electrode current collector tab, cut and raised from the aluminum foil, consists of an extremely thin and slender piece of foil, so that the mechanical strength thereof is rendered extremely low.
The above-described method of sealing the opening of the outer case results in the breakage of the positive electrode current collector tab when press-fitting the sealing plate into the opening in the case where, for example, the fit between the sealing plate and the outer case of the battery is tight, or when the battery as fabricated is inadvertently dropped to the floor, or when strong vibration, etc. is applied to the battery, and so forth, such that the positive electrode current collector tab is broken, thus resulting in electrical contact continuity based only on the contact condition between the outer case of the battery and the exposed part of the core body of the positive electrode. Accordingly, therefore, problems have arisen in that such electrical contact continuity is very unstable because of changes in internal resistance arising from change in contact pressure between the exposed part of the core body of the positive electrode and the inner face of the outer case, and in addition, the internal resistance of the battery increases.